How to turn a linked group into independent objects?
You can make objects instantiated by group to be real ones.

While group instance selected, execute Make Duplicates Real operator (or press Ctrl+Shift+A if in 2.79 or prior) to make objects from group as real ones. You can now deal with them as with meshes, because they already are. 

Then you can run Make Single User operator from Object menu (or press U > Object Data if in 2.79 or prior) to make these objects use their own mesh datablock (if there is needed possibilty to edit them individually).


While this will work, it has it downsides:

Pros


Make objects from group finally editable (sometimes may be useful)


Cons


Additional geometry will be added that can lower scene processing or rendering time.
Additinal objects will appear in Outliner (if objects were made to use their own mesh datablocks), so in case of there are many objects in the scene, you may want to avoid this so not to slow down 3D View.